1. Field of Technology
The present invention relates to an imaging apparatus using a MOS solid state imaging element, and to an imaging method.
2. Description of Related Art
Solid state imaging elements (image sensors) are used to convert light from a subject to an electric signal in digital cameras, video cameras, and other imaging devices used in applications for capturing and storing images. Solid state imaging elements mainly include CCD image sensors and MOS image sensors. CCD image sensors are more common today, however, because of such features as a wide dynamic range and isochronous imaging performance. Disadvantages to CCD image sensors include high power consumption and the need for different power supply voltages, and further significant improvement in these characteristics is not expected.
MOS image sensors have a narrower dynamic range and are more susceptible to noise than CCD image sensors, but recent research has reduced the performance gap between MOS and CCD image sensors. In addition, CMOS image sensors offer a number of advantages over CCD image sensors, including lower power consumption and random accessibility, that is, the ability to freely define the order in which the pixels are read. Furthermore, because CMOS image sensors can be manufactured using the same equipment and materials used to manufacture CMOS semiconductor devices, existing CMOS semiconductor fabrication plants can be used to manufacture CMOS image sensors, thereby reducing cost. Because of these numerous benefits, CMOS image sensors are increasingly common.
Solid state image sensors are used in such products as digital still cameras, video cameras, and cell phones, and produce still or video images through the following process.
(a) The electric signals acquired by the CCD or MOS image sensor are read pixel by pixel one line at a time from one end of the sensor, and temporarily stored in relatively inexpensive memory, such as SDRAM (synchronous DRAM).
(b) When reading and writing the first line to SDRAM ends, the process repeats to read the second line, then the third line, and so forth until one full frame has been stored in SDRAM.
(c) These signals are then read from SDRAM and passed to a signal processor for zoom processing to enlarge or reduce the image, and the data is again written to SDRAM.
(d) The processed data is then read from SDRAM and compressed to a format suitable for recording, such as JPEG image data, and the compressed image data is again written to SDRAM.
(e) The compressed data is then read quickly from SDRAM by DMA (direct memory access) control, for example, and output to external semipermanent storage.
When capturing motion picture images, however, the number of frames captured per second is an extremely important factor determining the quality and smoothness of the captured video. Based on recent data, capturing from 30 to as many as 60 frames per second (fps) is necessary. At 30 fps, 1/30 second can be used to capture each frame. At a VGA image size of 640×480 pixels, data for 307,200 pixels must be extracted. For a high definition image, image data for 1920×1080=2,073,600 pixels, that is, 6.75 times as many pixels in a VGA image, must be captured. Driving the image sensor at such high seed depends greatly upon the CMOS process, and is technically difficult.